1. Field of the Invention
The present invention relates to a power amplifying apparatus, and particularly to a power amplifying apparatus effectively applicable to a wireless transmitter such as a mobile communication system.
2. Description of the Related Art
As a communication system, there is, for example, a telephone system for automobiles. The automobile telephone system is composed of an exchanging station to be connected to a public telephone network through a local station as a superior station, a base station for setting a mobile wireless line between mobile stations as an automobile telephone station on the indication of the exchanger, and mobile stations as the automobile telephone including a controlling circuit driven by the indication from the exchanger. It is necessary for the communication system to control an output level of an electric power in the transmitter within the automobile telephone, in accordance with the distance between the base station and the automobile telephone. Therefore, the transmitter has a power amplifying apparatus in which the output electric power level can be switched, for example, 6 to 8 stages, thereby providing a suitable transmission electric power between the base station and the automobile telephone.
There is a prior art power amplifying apparatus in which an output electric power level is changed, such as "Bidirectional Field Type Voltage Controlled Amplifier (BDF-DVCA)" by Chiba et al., Singaku Review, Vol. 89, No. 250, Pages 7 to 12. The prior art power amplifying apparatus is constructed in such a way that the power amplifier composed of a plurality of amplifiers for amplifying transmission electric power, is controlled in a feedback system in accordance with the distance from the base station. In general, a conventional automobile power amplifier in a cellular system uses a C-class amplifier as an output amplifier in point of an efficiency of an electric power. In the case where such C-class amplifier is used as a linear amplifier in an analog cellular system, it is important to use an amplifier element in a saturation state in view of the electric power efficiency. Therefore, the C-class amplifier has been used for a feedback control for amplifying the electric power thereby increasing the efficiency of an electric power source.
However, according to the conventional technique for effecting the electric power control by using C-class amplifier as an output amplifier, if a feedback voltage of the C-class amplifier is much reduced in order to reduce the transmission power outputted from the power amplifier, the collector-base junction in the transistor composed of the C-class amplifier is reversed in comparison with a normal bias condition, thereby increasing a feedback capacitance and providing an unstable condition. Therefore, it is necessary to apply a feedback voltage within the range in which the C-class amplifier does not oscillated to make the unstable condition and further the power control at the low level side of the output power is limited thereby resulting in a narrow range of the power control. This is similar to the case that an FET transistor is used as the C-class amplifier. Moreover, according to the prior art there is the problem that the phase delay of a pulse eliminating filter is large and a loop oscillation is apt to be generated. Furthermore, in the prior art the collector voltage is changed, and therefore the feedback capacitance between the collector and the base thereof is changed thereby generating a conversion distortion in which the output phase is changed by an amplitude, in the conversion of an amplitude modulation (AM) to a phase modulation (PM). Moreover, it is difficult to control the output electric power over 50 dB or more.
Moreover, there is a prior art in which a constant voltage is applied to an output amplifier, and an AB-class amplifier is disposed at the front stage of the output amplifier used for a control, and a feedback control is effected with respect to the control amplifier. However, according to the power amplifying apparatus thus constructed, there are the following problems with respect to a linearity of an electric power control. FIG. 12 shows an output power characteristic in the prior art when the feedback voltage to be applied to the power source terminals of the power amplifier is changed. In the figure, the abscissa is a feedback voltage in a linear scale and the ordinate is an output power P0 in the logarithm scale. In the figure, the solid line Pl shows a practical control characteristic curve and the dotted line P2 shows an ideal linear curve. As is apparent from the figure, the output power characteristic can be divided into three areas of a non-linear area A, a linear area B and a saturation area C. The non-linear area A is one due to a limit of the isolation characteristic of each of the amplifying elements composing the power amplifier. Namely, even if for example the input power is made a minimum value or zero in view of the characteristic of the amplifier element, a leakage of signals to the output side is generated because of an isolation characteristic thereby not making the output power zero. The linear area B is one in which the output power is changed with a linearity with respect to the feedback voltage applied to the control amplifier. The saturation area C is one resulted from the ability limit of the amplifier element. For example, according to an automobile telephone in a digital cellular mobile communication system, it is important that the output power of the power amplifier is linearly changed with respect to the feedback voltage and further an electric power can be controlled over wide range of 50 dB. However, there are problems for the conventional power amplifying apparatus that the non-linearity in the non-linear area A upon small output and that the control range of the electric power is narrow because of a large maximum controllable electric power due to the limit caused by the isolation characteristic.
Furthermore, in a transmitter of the digital cellular mobile telephone applied with the conventional technique, there is used an amplitude-phase modulation system such as .pi./4 shift DQPSK (differential quadrature phase shift keying). In this case, since information is contained in not only a phase component but also an amplitude component, it is necessary to correctly amplify in fidelity also the amplitude component in a linear electric power amplifying apparatus. However, since the internal impedance of the power source for supplying an electric power to the amplifiers within the electric power amplifying apparatus and the impedance of the power source line are actually not zero, the voltage deviation corresponding to the output power envelope from the power amplifier is generated also at the power source terminal for supplying the electric power to the power amplifier. Upon generation of such deviation, the output signal is modulated by the deviation thereby undesirably expanding a transmission spectrum. FIGS. 13a and 13b show a deterioration due to the deviation of the source voltage, and FIG. 13a is a signal space view of .pi./4 DQPSK, and FIG. 13b shows the expansion of spectrum with the abscissa of frequency F and the ordinate of an electric power level. When the conversion distortion in the conversion of the amplitude modulation to the phase modulation in the power amplifier is generated due to the generation of the deviation of the source voltage, the deviation of signal points S as shown in FIG. 13a is produced. As a result, the edge portions at both sides of the transmission channel (e.g. 30 KHz) is shifted upwardly as shown by the arrow in FIG. 13b, and the spectrum is expanded, thereby causing a radio interference with respect to the adjacent channels. To avoid such deviation of the source voltage, it is considered to provide a bypass capacitor with large capacitance for the electric power source.
However, in the mobile telephone of the digital system, since a time divisional multiplex access system (TDMA system) is normally used for the purpose of increasing the utilization efficiency of channel, a burst control is effected. Therefore, the power amplifier also performs an ON-OFF control of the power source voltage to effect the burst control. FIG. 14 shows an example of the burst power control for the North America System. According to this burst power control, the output power P0 is set ON-state during e.g. 6.66 ms and then OFF-state during e.g. 6.66.times.2 ms in order to effect a communication cf one telephone. Since the TDMA system is utilized with high precision in the burst power control, it is necessary that a predetermined electric power level is achieved within a predetermined ramp-up time ta at the raising position of the output power P0. However, if the bypass capacitor with large capacitance is used to avoid the deviation of the power voltage, the response time necessary for the ON and OFF operation of the power voltage is increased, and therefore the ramp-up time ta of the output power P0 exceeds a predetermined time. Since the time divisional multiplex access system is used for the digital mobile telephone, it is substantially impossible to avoid the expansion of the spectrum by increasing the capacitance of the bypass capacitor.